This invention relates to a circuit arrangement for detecting missing pulses in a variable frequency source.
It is often necessary to monitor a frequency source in a frequency sensitive system, such as a motor control system, wherein it is desirable to perform some control operation within the time frame of a single period of the frequency source. Missing pulse detectors have been used for this purpose to detect the absence of a pulse in a frequency source. However, typical missing pulse detection circuits have been disadvantaged with respect to the range of source frequencies over which they are capable of operating. For example, detectors which generate a delayed reference pusle in response to an input source pulse cannot operate unless the delay time corresponds to the period of the source signal. Thus, such a detector can operate only over a narrow range of source frequencies. Similarly, a heart-beat monitoring system using a rate matching technique has a limited frequency response because of instabilities which result if the souce frequency is less than one-half of the inverse of the system response time. Another type of missing pulse detection circuit utilizes a frequency doubler with a variable period retriggerable one-shot. The disadvantages of such a circuit are reduced frequency response due to the frequency doubler and temperature instabilites due to the one-shot R-C timing circuit elements.